Humans have rarely encountered an enemy as devastating as the bacterium Yersinia pestis. Between 1347 and 1351 it sparked the Black Death, an infection carried by fleas that spread rapidly across Europe, killing around 50 million people.
Now scientists have uncovered some of the genetic secrets of the plague, thanks to DNA fragments drilled from the teeth of victims buried in a graveyard in East London.
The researchers say that all current strains circulating in the world are directly related to the medieval bacterium. And while the infection still kills 2,000 people globally every year, it's much less of a threat because our immune systems have adapted to it.
The scientists say the techniques used to rebuild the genome of the Black Death could now be used to reconstruct other ancient pathogens.